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Interaction of methyltin(IV) compounds with carboxylate ligands.

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Research Article
Received: 16 July 2007
Revised: 25 September 2007
Accepted: 2 October 2007
Published online in Wiley Interscience: 18 December 2007
(www.interscience.com) DOI 10.1002/aoc.1346
Interaction of methyltin(IV) compounds with
carboxylate ligands. Part 2: formation
thermodynamic parameters, predictive
relationships and sequestering ability
Concetta De Stefanoa , Antonio Gianguzzab∗ , Ottavia Giuffrèa,
Alberto Pettignanob and Silvio Sammartanoa
Thermodynamic data of mono-, di- and tri-methyltin(IV)-carboxylate complexes (acetate, malonate, succinate, oxydiacetate, diethylenetrioxydiacetate, malate, citrate, 1,2,3-tricarballylate, 1,2,3,4-butanetetracarboxylate, 1,2,3,4,5,6benzenehexacarboxylate) in aqueous solution are reported at t = 25 ◦ C and I = 0 mol l−1 . Thermodynamic parameters
obtained were analysed to formulate empirical predictive relationships as a function of different parameters, such as the
number of carboxylate groups of the ligand and the charge of the alkyltin(IV) cation. Sequestration diagrams of citrate and
c 2007 John
1,2,3-tricarballylate towards alkyltin(IV) cations at different pH values are also reported and discussed. Copyright Wiley & Sons, Ltd.
Keywords: methyltin(IV) complexes; thermodynamic parameters; calorimetry; empirical relationships; speciation in aqueous solution
Introduction
30
Over the last decade, our research group had carried out an
extensive study on the speciation of alkyltin(IV) cations in aqueous
solution.[1 – 12] First, the hydrolysis processes of mono-, di- and
tri-organotin cations in various ionic media and at different
temperatures and ionic strengths were studied;[1 – 3] subsequently,
the speciation of organotins in aqueous media, by simulating
the composition of natural waters,[4 – 7] and the interactions
with ligands of biological and environmental interest, such as
carboxylate,[8 – 12] were investigated. In the first part[12] of this
short series, we reported the stability data of complexes of mono-,
di- and tri-methyltin(IV) cations with mono-, di-, tri-, tetra- and
hexacarboxylate ligands obtained potentiometrically ([H+ ]-glass
electrode) at t = 25 ◦ C and I → 0 mol l−1 . In order to improve
the thermodynamic picture and to obtain more information on
the nature and the extent of these interactions, it was necessary
to determine enthalpy and entropy values for these complexation
equilibria. In fact, for reactions in aqueous solution, formation
enthalpies of the species metal–ligand give information on the
breaking and formation of chemical bonds involving the metal
ion, the ligand and the solvent. Entropies measure the order
in the system and show the change in solvation between the
uncomplexed and complexed states of metal ion and ligands.
Some papers[13 – 19] have appeared in the literature dealing
with equilibria and structural aspects of complexes formed
by the interaction of alkyltin(IV) cations with O-donor ligands,
including amino acids. Only one of these gives thermodynamic
data regarding dimethyltin(IV)-acetate, -malonate and -succinate
species.[15]
In this paper, enthalpy and entropy data of mono-, diand tri-methyltin(IV)-carboxylate complex species are reported.
The measurements have been carried out by calorimetry at
Appl. Organometal. Chem. 2008; 22: 30–38
25 ◦ C in aqueous solution and the following ligands have been
investigated: acetate (ac), malonate (mal), malate (mala), succinate
(succ), oxydiacetate (oda), diethylenetrioxydiacetate (toda), 1,2,3tricarballylate (tca), citrate (cit), 1,2,3,4-butanetetracarboxylate
(btc) and 1,2,3,4,5,6-benzenehexacarboxylate (mellitate, mlt).
Structures of all these ligands with the relative abbreviations
are reported in Fig. 1. The large number of data (G0 , H0 and
TS0 ) collected for the systems investigated and for the complex
species formed (25 and 90, respectively), were analysed for the
purposes of prediction, to find empirical relationships as a function
of different parameters, such as the number of carboxylate groups
of the ligand and the charge of the alkyltin(IV) cation.
Experimental
Materials
Monomethyl-, dimethyl- and trimethyl-tin(IV) compounds were
used as chloride salts. The solutions were prepared from Aldrich
commercial products twice re-crystalized before use. Carboxylate
ligands (Fluka or Aldrich) were used without further purification.
Their purity was checked by potentiometric titrations. Hydrochloric
∗
Correspondence to: Antonio Gianguzza, Dipartimento di Chimica Inorganica
e Analitica ‘‘Stanislao Cannizzaro’’, Università di Palermo, Viale delle Scienze,
90128 Palermo, Italy. E-mail: giang@unipa.it
a Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica,
Università di Messina. Salita Sperone 31, Villaggio S. Agata, 98166 Messina,
Italy
b Dipartimento di Chimica Inorganica e Analitica ‘Stanislao Cannizzaro’,
Università di Palermo, Viale delle Scienze, 90128 Palermo, Italy
c 2007 John Wiley & Sons, Ltd.
Copyright Interaction of methyltin(IV) compounds with carboxylate ligands: 2
O
O
O
O
HO
HO
OH
OH
OH
OH
O
malic acid (mala)
malonic acid (mal)
acetic acid (ac)
O
O
O
HO
O
OH
O
HO
OH
oxydiacetic acid (oda)
HO
O
HO
O
HO
O
OH
propane tri-carboxylic acid
(tricarballylic acid, tca)
succinic acid (succ)
O
O
O
O
HO
OH
O
HO
O
OH
O
OH
citric acid (cit)
O
diethylenetrioxydiacetic acid (toda)
HO
O
HO
O
O
O
O
HO
HO
OH
HO
OH
OH
O
HO
O
O
butanetetracarboxylic acid (btc)
O
HO
O
benzene hexacarboxylic acid
(mellitic acid, mlt)
Figure 1. Structures of ligands.
acid and sodium hydroxide solutions were prepared by diluting
concentrated Fluka ampoules and standardizing against sodium
carbonate and potassium hydrogen phthalate, respectively. All
solutions were prepared with analytical-grade water (resistivity =
18 M cm), using grade A glassware.
Equipment and procedure
Appl. Organometal. Chem. 2008; 22: 30–38
(CH3 )SnCl3
(CH3 )2 SnCl2
(CH3 )3 SnCl
C(CH3 )x SnCl4−x a
CL a
Ia,b
Ntit c
0.001–0.003
0.001–0.004
0.003–0.010
0.085–0.26
0.05–0.20
0.16–0.3
0.008–0.010
0.004–0.013
0.010–0.017
28
24
28
a Concentrations in mol l−1 ; b range of ionic strength; c total number of
titrations.
the calculation of the hydrolysis enthalpies of monomethyltin(IV)
cation, a solution containing this species (1–3 mmol l−1 ) was
titrated with standard NaOH.
Calculations
The following computer programs were used: LIANA,[20] a linear
and non-linear least squares program, to calculate relationships of
dependence of thermodynamic parameters on charge; ES5CM,[21]
to analyse calorimetric titration data, to deal with data obtained
c 2007 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
31
Calorimetric measurements were performed by titrating 50 ml
of the solution containing the alkyltin(IV) cation under study
with the carboxylate sodium salt at 25.000 ± 0.001 ◦ C, by means
of a model 450 Tronac Isoperibol Titration calorimeter, coupled
with a Keithley 196 system Dmm digital multimeter. Details of
experimental conditions are reported in Table 1. The titrant was
delivered by a 2.5 ml capacity Hamilton syringe, model 1002TLL. A
computer program was used for the acquisition of the calorimetric
data. The system accuracy was checked by titrating a Tris
[tris-(hydroxymethyl)amino-methane] buffer with HCl. The heat
of dilution was measured before each experiment. The accuracy
of calorimetric apparatus was Q ± 0.008 J and v ± 0.001 cm3 .
The protonation enthalpies of toda were obtained titrating a
solution of its sodium salt (2.5–5 mmol l−1 ), with standard HCl. For
Table 1. Experimental conditions for calorimetric measurements
(t = 25 ◦ C)
C. De Stefano et al.
Table 2. Hydrolysis enthalpiesa
I = 0 mol l−1 and t = 25 ◦ C
of
(CH3 )x Sn(4−x)
1–1
1–2
1–3
1–4
2–2
2–3
2–5
at
Results and Discussion
Enthalpy and entropy changes
H0
pq
cations
pq
(CH3 )Sn3+b
(CH3 )2 Sn2+c
(CH3 )3 Sn+d
−24 ± 2
−11 ± 3
15 ± 3
51 ± 3
–
–
9±4
33.1
62.1
97.7
–
60
84
–
25.8
80.8
–
–
–
–
–
H0 pq refers to the reaction: pM + qH2 O = Mp (OH)q + qH and is
expressed in kJ mol−1 ; b this work; c Foti et al.[4] ; d De Stefano et al.[2]
The overall enthalpy changes, H0 , determined by calorimetric
measurements, are shown in the Tables 4–6, together with
G0 and TS0 values. Equilibrium thermodynamic parameters,
reported in Tables 7–9, were calculated according to the following
reactions:
a
in variable ionic strength conditions and to perform corrections
to I = 0 mol l−1 . At I < 0.05 mol l−1 , the contribution of this
extrapolation procedure to the total error is less than 0.2 kJ mol−1 .
Concentrations and thermodynamic parameters are given in the
molar scale. Errors are given as standard deviations. The overall
formation equilibria are expressed as (charges omitted)
pM + qL + rH = Mp Lq Hr
(1)
where r > 0 for protonated species and r < 0 for hydrolytic
species.
The hydrolysis enthalpies of di- and tri-methyltin(IV) have been
reported in previous papers,[2,4] as well as the protonation enthalpies of the carboxylate ligands under investigation, also
in relation to their association with metal ions.[22 – 25] Protonation enthalpies of toda and mlt and hydrolysis enthalpies of
monomethyltin(IV) are determined and reported for the first time
in this work. Hydrolysis enthalpies of mono-, di- and trimethyltin(IV)
cations and protonation enthalpies of carboxylates used in this
work are reported in Tables 2 and 3.
M + LHi
=
MLHi
(2)
M(OH)j + L
=
ML(OH)j
(3)
L + ML
=
ML2
(4)
M + ML
=
M2 L
(5)
As organotin(IV) compounds are considered Lewis acids having
different hardnesses [CH3 Sn3+ (CH3 )2 Sn2+ > (CH3 )3 Sn+ ], the
interaction with hard base donors such as oxygen of carboxylate
ligands is mainly driven by a favourable entropic factor, typical for
hard–hard interactions, where free energy mainly results from the
entropy gain, due to the change of orientation of water molecules
of hydration.[26] On the whole, the results in Tables 4–9 confirm
that the major contribution to the stability of these complex
species is due to the entropic term.
On the other hand, enthalpy values are not too high and
they are mostly of endothermic nature. This indicates that,
as dehydration of the metal requires energy to break the
cation–water and water–water bonding of the hydration layers,
the energy required to break bonds with water is higher than
the energy released during the formation of metal–ligand
bonds.[27] This endoergic dehydration effect overcomes exoergic
metal–oxygen interactions, except for some complex species of
monomethyltin(IV) with mal, succ, toda, tca, cit and btc and some
others of di- and trimethyltin(IV) with cit.
Moreover, thermodynamic results show that other factors
influence the interaction: (i) charge effects – an increase in entropy
values with increasing ligand charge, as well a decrease with
decreasing cation charge is shown; (ii) structural effects – the
presence of other potentially binding groups (alcoholic or ethereal)
induces a significant lowering of TS0 values. For example in the
(CH3 )2 Sn2+ -bicarboxylate ligand systems, for the MLOH species,
overall TS0 = 55.4, 49.7, 22 and 14 kJ mol−1 for mal, succ,
oda and toda, respectively; in the (CH3 )2 Sn2+ -tricarboxylate
Table 3. Protonation enthalpiesa of carboxylate ligands, at I = 0 mol l−1 and t = 25 ◦ C
H0 j
Ligand
ac
mal
succ
oda
toda
mala
cit
tca
btc
mlt
a
j=1
j=2
0
5
0.7
10
4.2 ± 0.1b
0
1.6
3.4
6.6
13.0 ± 0.1b
4
−1.7
9.3
7.8 ± 0.1b
−2
−2.5
3.7
5.7
10.6 ± 0.1b
j=3
7.9
−0.9
4.4
8.2 ± 0.1b
j=4
0.4
5.6 ± 0.1b
j=5
3.4 ± 0.1b
j=6
Reference
1.0 ± 0.1b
[22]
[22]
[23]
[23]
this work
[22]
[24]
[25]
[25]
this work
32
H0 j refers to the equilibrium reaction: Lz− + jH+ = Hj L(j−z) and are expressed in kJ mol−1 ; b ±s = standard deviation.
www.interscience.wiley.com/journal/aoc
c 2007 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2008; 22: 30–38
Interaction of methyltin(IV) compounds with carboxylate ligands: 2
Table 4. Overall thermodynamic parametersa for the formation of
complex species in the CH3 Sn3+ -carboxylate ligands (L) systems at
I = 0 mol l−1 and t = 25 ◦ C
L
pqrb
−G0
H0 ± sc
TS0
ac
11–1
12–1
11–2
21–5
11.9
19.4
−3.2
−24.5
2.1 ± 0.5
1.6 ± 0.5
1.2 ± 0.5
22 ± 2
14
21
−2
−25
mal
110
11–1
11–2
49.1
33.0
−0.4
−1.4 ± 0.4
−4.8 ± 0.3
14.4 ± 1.0
47.7
28.2
14
succ
110
11–1
11–2
50.8
31.1
1.9
−9 ± 1
13.1 ± 0.8
10.3 ± 0.6
41.8
44.2
12.2
11–1
11–2
11–3
27.3
4.6
−22.3
0.6 ± 0.2
15.4 ± 0.4
15.8 ± 0.9
27.9
20
−6.5
11–1
11–2
11–3
22.4
3.4
−22.4
−2.7 ± 0.9
12 ± 1
−9.5 ± 0.9
19.7
15.4
−31.9
11–1
11–2
11–3
36.2
14.5
−23.4
2.8 ± 0.7
2.5 ± 0.8
1.4 ± 0.8
tca
110
111
11–1
11–2
66.7
85.6
41.8
8.2
0.8 ± 0.5
−8.6 ± 0.7
4.6 ± 0.5
17.4 ± 0.9
67.5
77
46.4
25.6
cit
110
11–1
11–2
11–3
73.1
52.8
21.9
−20.3
−26.5 ± 0.4
−19.2 ± 0.4
−12.1 ± 0.8
9.8 ± 0.4
46.6
33.6
9.8
−10.5
btc
110
111
112
11–1
11–2
77.5
101.6
118.9
47.5
9.4
−3.8 ± 0.5
−4.1 ± 0.5
−4.9 ± 0.8
−1.9 ± 0.4
3.2 ± 0.9
73.7
97.5
114
45.6
12.6
oda
toda
mala
a
c
39
17
−22
Values in kJ mol−1 ; b indexes refer to the equilibrium reaction (1);
s = standard deviation.
Appl. Organometal. Chem. 2008; 22: 30–38
The formation reactions can also be expressed with the following
equilibria (charges are omitted for simplicity):[10]
M(OH) + L + nH
=
M(OH)LHn
M(OH)LHn
=
M(OH)LHn−1 + H
(6)
βn
Kn
(7)
Tables 10 and 11 show average values of stability constants and
entropy, respectively calculated using the data with the same
nH value, according to the equilibrium 6. On the whole, these
results indicate that the stability and the entropy of methyltin(IV)carboxylate species is strictly dependent on nH , as expressed by
the following equations (R = correlation coefficient).
log βn
=
7.8 + 4.9 nH
with
R = 0.9997 for CH3 Sn3+
log βn
=
3.5 + 4.9 nH
with
R = 0.952 for (CH3 )2 Sn2+
log βn
=
1.1 + 7.0 nH
with
R = 0.9645 for (CH3 )3 Sn+
Table 5. Overall thermodynamic parametersa for the formation of
complex species in the (CH3 )2 Sn2+ -carboxylate ligands (L) systems at
I = 0 mol l−1 and t = 25 ◦ C
L
pqrb
−G0
H0 ± sc
TS0
ac
110
120
11–1
17.2
30
−5.3
2.8 ± 0.5
5 ± 0.5
5.3 ± 1.0
20
35
0
mal
110
111
11–1
120
31.0
44.5
0.1
41.2
27.1 ± 0.2
17 ± 2
55.3 ± 0.4
8.8 ± 1.5
58.1
61.5
55.4
50
succ
110
111
11–1
28.4
48.9
0.3
30.1 ± 0.5
15 ± 2
49.4 ± 0.4
58.5
63.9
49.7
oda
110
111
11–1
34.3
43.9
−9.1
10.7 ± 0.1
24.1 ± 1.0
31.1 ± 1.5
45.0
68
22
toda
110
111
11–1
26.9
40.3
−2.9
29.1 ± 0.2
19.7 ± 0.5
16.9 ± 1.5
56
60
14
tca
110
111
112
11–1
38.2
63.4
82.1
5.8
44.7 ± 0.6
30.7 ± 0.8
26 ± 2
45.1 ± 0.7
82.9
94.1
108
50.9
cit
110
111
11–1
220
21–1
21–2
44.0
70.5
10.6
99.5
48.2
22.0
5.6d
5.0
−2.6
45
54.5
59.5
btc
110
111
112
113
11–1
46.8
76.1
99.7
116.4
10.3
35.6 ± 0.6
27.3 ± 0.7
15.0 ± 1.0
18.6 ± 1.5
30.8 ± 0.9
a
c
50
75
8
144
103
81
82.4
103.4
114.7
135
41.1
Values in kJ mol−1 ; b indexes refer to the equilibrium reaction (1);
s = standard deviation; d data from Cardiano et al.[11]
c 2007 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
33
ligand systems, for the ML species, overall TS0 = 82.9 and
50 kJ mol−1 for tca and cit, respectively; in the (CH3 )3 Sn+ bicarboxylate ligand systems, for the MLH species, overall
TS0 = 66.1, 69, 52 and 49 kJ mol−1 for mal, succ, oda and
toda, respectively.
Using log β and H0 values at t = 25 ◦ C, it is possible to
calculate stability constants at any temperature using the Clarke
and Glew equation,[28] which takes into account the temperature
dependence of stability constants. As an example we chose the
system dimethyltin(IV)-succinate; after calculating the stability
constants of the species at t = 5 and 45 ◦ C, we plotted the
speciation diagram shown in Fig. 2. As can be seen, with the
increasing of the temperature beyond the growth of the formation
percentage of the species, there is a shift of the maximum of the
curve to lower pH values.
Empirical relationships
C. De Stefano et al.
Table 6. Overall thermodynamic parametersa for the formation of
complex species in the (CH3 )3 Sn+ – carboxylate ligands (L) systems at
I = 0 mol l−1 and t = 25 ◦ C
Table 7. Thermodynamic parameters for the formation of complex speciesa in the CH3 Sn3+ – carboxylate ligands (L) systems at
I = 0 mol l−1 and t = 25 ◦ C
L
pqrb
−G0
H0 ± sc
TS0
L
Species
log K
−G0b
H0b
TS0b
mal
110
111
11–1
11–2
15.6
44.1
−21.1
−86.8
8.4 ± 0.6
22 ± 1
23 ± 1
−95 ± 3
24
66.1
1.9
−8
ac
MLOH
ML(OH)2
3.6
2.9
20.5
16.6
11.4
8.7
31.9
25.3
mal
13.5
41.0
10.7 ± 0.3
28 ± 3
24.2
69
ML
MLOH
ML(OH)2
8.6
7.3
3.4
49.1
41.6
19.4
4.9
12.2
12.3
54.0
53.8
31.7
succ
ML
MLOH
ML(OH)2
8.9
7.0
3.8
50.9
39.7
21.7
0.9
20.7
17.1
51.8
60.4
38.8
mala
MLOH
ML(OH)2
ML(OH)3
7.8
6.0
5.0
44.8
34.2
28.5
7.5
7.5
5.6
52.3
41.7
34.1
oda
MLOH
ML(OH)2
ML(OH)3
6.3
4.3
5.2
35.8
24.4
29.6
16.7
17.9
1.5
52.5
42.3
31.1
succ
110
111
oda
110
111
11–1
12.0
32.9
−25.0
22.9 ± 0.6
19.1 ± 1.0
35 ± 3
34.9
52
18
toda
110
111
11–1
11–2
12.9
34.1
−23.5
−90.2
7.8 ± 0.1
14.9 ± 0.5
26.5 ± 1.0
59 ± 3
20.7
49
3
−31
110
111
112
18.7
50.4
73.6
11.5 ± 0.3
18 ± 1
17 ± 3
30.2
68.4
91
toda
110
111
112
19.2
50.8
75.8
−2.7 ± 0.3
7.0 ± 1.0
3.2 ± 1.2
16.5
57.8
79
MLOH
ML(OH)2
ML(OH)3
5.7
4.0
5.2
32.7
23.1
29.5
16.9
17.5
−10.5
49.6
40.6
19.0
tca
110
111
112
210
21.1
58.5
87.6
39.5
5.0 ± 0.4
26 ± 1
10.4 ± 1.1
−5 ± 1
26.1
84.5
98
34.5
ML
MLH
MLOH
ML(OH)2
11.7
8.5
8.8
4.9
66.7
48.6
50.4
28.0
1.8
0.8
15.5
21.4
68.5
49.4
65.9
49.4
cit
110
111
112
210
36
73.4
102.6
52.7
11 ± 1
1.6 ± 0.7
−1.6 ± 0.8
31 ± 2
ML
MLOH
ML(OH)2
ML(OH)3
12.8
10.8
7.3
5.5
73.1
61.4
41.7
31.6
−15.0
2.5
4.4
7.1
58.1
63.9
46.1
38.7
btc
ML
MLH
MLH2
MLOH
ML(OH)2
13.6
10.6
7.8
9.8
5.1
77.5
60.7
44.7
56.1
29.1
−1.5
−0.7
2.6
12.9
17.9
76.0
60.0
47.3
69.0
47.0
tca
cit
btc
mlt
a
c
47
75
101
84
Values in kJ mol−1 ; b indexes refer to the equilibrium reaction (1);
s = standard deviation.
a
MLOH
60
5°C
25°C
45°C
% (CH3)2Sn2+
Referring to the equilibria (2) and (3); b in kJ mol−1 .
groups of the ligand (ncarbox ) and the charge of the alkyltin(IV)
cation (zcation ), have been calculated:
40
ML
Y = F1 + F2 nH + F3 ncarbox
(8)
where for log β
20
MLH
0
2
3
4
5
pH
6
7
=
−2.8(±0.3) + 0.59(±0.07) z2 cation
F2
=
6.0(±0.1) − 0.21(±0.02) z2 cation
F3
=
1.43(±0.07) + 0.04(±0.02) z2 cation
8
and for TS0
Figure 2. Speciation diagram of the species formed by dimethyltin(IV)
(M) with succinate (L) at t = 5, 25 and 45◦ , CM = 1 mmol l−1 ; CL =
5 mmol l−1 .
34
By considering all stability and entropy data, according to the
equilibria (6) and (7), the following correlations, which, besides
the nH parameter, take into account the number of carboxylate
www.interscience.wiley.com/journal/aoc
F1
F1
=
−7(±7) + 5(±1) z2 cation
F2
=
29(±4) − 1.1(±0.5) z2 cation
F3
=
2(±2) + 0.9(±0.4) z2 cation
Figures 3 and 4 show the plots of log βn and TSn 0 values vs log βn and TSn 0 values calculated with equation (8).
c 2007 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2008; 22: 30–38
Interaction of methyltin(IV) compounds with carboxylate ligands: 2
Table 8. Thermodynamic parameters for the formation of complex speciesa in the (CH3 )2 Sn2+ – carboxylate ligands (L) systems at
I = 0 mol l−1 and t = 25 ◦ C
L
Species
log K
−G0b
H0b
TS0b
ac
ML
ML2
MLOH
3.0
2.2
1.9
17.2
12.8
11.0
8.3
7.6
6.6
mal
ML
MLH
MLOH
ML2
5.4
2.1
2.9
1.8
31.0
12.0
16.3
10.2
ML
MLH
MLOH
5.0
2.9
2.9
ML
MLH
MLOH
succ
oda
toda
tca
cit
btc
a
Table 9. Thermodynamic parameters for the formation of complex speciesa in the (CH3 )3 Sn+ – carboxylate ligands (L) systems at
I = 0 mol l−1 and t = 25 ◦ C
L
Species
log K
−G0b
H0b
TS0b
25.5
20.4
17.6
mal
ML
MLH
MLOH
2.7
2.0
2.4
15.6
11.6
13.9
10.0
12.3
2.1
25.6
23.9
16.0
15.8
13.7
16.3
10.0
46.8
25.7
32.6
20.2
succ
ML
MLH
2.4
1.5
13.6
8.8
11.6
17.2
25.2
26.0
oda
28.4
16.8
16.6
19.3
13.0
13.4
47.7
29.8
30.0
ML
MLH
MLOH
2.1
1.4
1.8
12.0
8.0
10.0
18.1
6.7
6.6
30.1
14.7
16.6
toda
6.0
3.3
1.3
34.3
19.0
7.2
8.2
8.5
13.0
42.5
27.5
20.2
ML
MLH
MLOH
2.3
1.7
2.0
12.9
9.9
11.5
10.9
5.8
6.0
23.8
15.7
17.5
tca
ML
MLH
MLOH
4.7
2.8
2.4
26.9
16.0
13.5
18.3
8.6
10.0
45.2
24.6
23.5
ML
MLH
MLH2
3.3
2.3
1.5
18.8
13.4
8.4
13.4
13.4
9.5
32.2
26.8
17.9
cit
ML
MLH
MLH2
MLOH
6.7
4.6
3.0
3.9
38.2
26.4
17.0
22.1
23.5
17.9
16.0
13.1
61.7
44.3
33.0
35.2
ML
MLH
MLH2
3.4
2.5
2.1
19.2
14.2
12.0
5.3
6.0
9.4
24.5
20.2
21.4
btc
ML
MLH
MLOH
7.7
5.9
4.7
44.0
33.8
26.9
6.5
6.3
−0.7
50.5
40.1
26.2
ML
MLH
MLH2
M2 L
3.7
3.1
2.3
3.2
21.1
17.6
13.3
18.4
13.2
19.1
15.4
−0.1
34.3
36.7
28.7
18.3
mlt
ML
MLH
MLH2
MLH3
MLOH
8.2
6.2
4.5
2.9
4.7
46.8
35.2
25.5
16.3
26.6
21.4
16.9
10.7
13.8
8.7
68.2
52.1
36.2
30.1
35.3
ML
MLH
MLH2
M2 L
6.3
5.0
3.7
2.9
36.0
28.6
20.9
16.7
35.3
32.3
29.7
26.8
71.3
60.9
50.6
43.5
a
Referring to the equilibria (2) and (3); b in kJ mol−1 .
Referring to the equilibria (2) and (3); b in kJ mol−1 .
Table 10. Average values of stability constantsa for species with
different nH values
The corresponding fits are fairly good, particularly for log βn , as
indicated by correlation coefficients (R = 0.978 and 0.903 for
log βn and TSn 0 , respectively), but there is a greater dispersion
of data for the latter, which probably indicates a higher specificity
of TS0 with respect to G0 .
Sequestering power of 1,2,3-propanetricarboxylate and
citrate towards alkyltin(IV)
As stability constants are not enough by themselves to indicate
the sequestering capacity of a ligand towards a metal, it
can be expressed by the function (%) vs pL, where (%)
is the total percentage of alkyltin(IV) cation complexed and
pL = − log[L]tot . Since this function is a typically sigmoidal curve
(or a dose–response curve), increasing rapidly over a relatively
small change in concentration, we can use the Boltzmann-type
equation[29] (with asymptotes of 100 for pL → −∞ and 0 for
pL → +∞):
(%) = 100 ×
1
1 + e(pL−pL50 )/S
(9)
CH3 Sn3+√
log β n ± s/ mc
4
3
2
1
0
−1
−2
22.3
17.9 ± 1.4
12.6 ± 1.0
7.9 ± 0.6
3.0 ± 0.5
−2.35 ± 0.13
2+
(CH3 )2 Sn√
log β n ± s/ mc
23.26
18.8 ± 1.5
12.6 ± 1.0
9.1 ± 0.5
3.2 ± 0.5
(CH3 )3 Sn√+
log β n ± s/ mc
21.0 ± 1.2
14.6 ± 0.8
9.4 ± 0.5
2.1 ± 0.2
−9.4 ± 0.3
Referred to the equilibrium reaction (6); b nH = number of protons in
the complex species; c m = total number of stability data on species
having equal nH .
a
where pL50 and S are empirical parameters which define the ligand
concentration necessary to sequester 50% of metal ion, while S
is a measure of the slope in flex of the function (%) vs pL. The
pL50 parameter is very useful because it gives a representation of
the binding ability of a ligand (L) towards a specific cation in the
conditions investigated.
Figures 5(a–c) and 6(a, b) show sequestration diagrams of
citrate and 1,2,3-propanetricarboxylate, respectively, towards
c 2007 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
35
Appl. Organometal. Chem. 2008; 22: 30–38
−1
nH b
C. De Stefano et al.
Table 11. Average values of entropya for species with different nH
values
20
logβn
nH
10
b
4
3
2
1
0
−1
−2
0
-10
-10
0
10
20
30
logβncalc
CH3 Sn3+
√
0
TSn ± s/ mc
147
120 ± 10
88 ± 6
68 ± 3
48 ± 2
15 ± 6
(CH3 )2 Sn2+
√
0
TSn ± s/ mc
(CH3 )3 Sn+
√
0
TSn ± s/ mc
118
94 ± 3
58 ± 6
45 ± 6
18 ± 7
101 ± 5
74 ± 4
37 ± 3
17 ± 5
−10 ± 11
Referred to the equilibrium reaction (6); b nH = number of protons in
the complex species; c m = total number of stability data on species
having equal nH .
a
Figure 3. log βn values according to the equilibrium 6 vs log βn values
calculated with equation (8).
Table 12. Empirical parameters of equation (9)a
160
pH = 5
pL50
Ligand
pH = 6
pL50
T∆Sn0
120
80
cit
tca
6.150
4.063
40
cit
tca
3.962
2.773
0
cit
tca
2.434
2.177
0
40
80
120
pH = 8
pL50
CH3 Sn3+
6.309
3.991
(CH3 )2 Sn2+
3.598
2.626
(CH3 )3 Sn+
2.685
2.544
5.613
2.004
a
With S [slope of the function of equation (9)] = 0.434 for all systems
at any pH value.
T∆Sn0calc
Figure 4. TSn 0 values (kJ mol−1 ) according to the equilibrium (6) vs
TSn 0 values (kJ mol−1 ) calculated with equation (8).
alkyltin(IV) cations, at CRSn = 10−8 mol l−1 and t = 25 ◦ C and
at different pH values. The pL50 values referring to citrate
and 1,2,3-propanetricarboxylate ligands toward mono-, di- and
trimethyltin(IV) cations are reported in Table 12. At the three pH
values examined, the trend of pL50 is mono-di-> trimethyltin(IV)
cation. At the same pH value, the binding ability of citrate is
significantly higher than that of 1,2,3-propanetricarboxylate. At
pH = 5, pL50 cit − pL50 tca = 2.087, 1.189, 0.257; at pH = 6,
pL50 cit − pL50 tca = 2.318, 0.972 and 0.141 for mono-, di- and
trimethyltin(IV) cation, respectively. The pL50 value decreases with
the increasing of the pH from 5 to 8, because the competition of
the hydrolysis of alkyltin(IV) cations is relevant. This is particularly
evident for trimethyltin–carboxylate systems where (CH3 )3 Sn+
cation is present practically only as hydrolytic species over pH = 6.
Correlation with the stability of different metal ions
36
In the first part[12] of this short series, our stability data for species
of mono- and di-methyltin(IV) cations with carboxylate ligands
were compared with those published in the literature relative
to the systems containing the same ligands with other metal
ions having the same charge (tables 7 and 8 of the first part),
www.interscience.wiley.com/journal/aoc
Table 13. Metal complexes of acetate butanetetracarboxylate at I =
0 mol L = 1 and t = 25 ◦ C.
log β
Species
(CH3 )2 Sn2+
Cu2+
Ni2+
Zn2+
Co2+
Pb2+
Ca2+
M(ac)
M(btc)
3.01
8.20
2.21
–
1.44
–
1.97
–
1.38
–
2.58
–
1.18
4.50
at t = 25 ◦ C, at various ionic strengths. Here we considered
the stability constants regarding only the ML species of table 8
of the first part,[12] recalculated at I = 0 mol l−1 , according
to the equation reported in Daniele et al.[30] The complexes
considered are formed by divalent cations, such as Cu2+ , Ni2+ ,
Zn2+ , Co2+ , Pb2+ and Ca2+ with several di- and tri-carboxylate
ligands (oda, toda, mal, succ, tca and cit). In the calculations, the
constant values from databasescited in Pettit and Powell[31] and
Martell[32] regarding acetate and butanetetracarboxylate ligands
at t = 25 ◦ C and I = 0 mol l−1 have been also considered
(Table 13).
We fitted all these data to the equation:
log K(CH3 )2 Sn2+ −L = a + b log KM2+ −L
c 2007 John Wiley & Sons, Ltd.
Copyright (10)
Appl. Organometal. Chem. 2008; 22: 30–38
Interaction of methyltin(IV) compounds with carboxylate ligands: 2
(a)
100
100
80
80
60
60
Σ%
Σ%
(a)
40
40
20
20
0
0
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
pCcit
6
7
8
9
5
6
7
8
9
(b)
100
100
80
80
60
60
Σ%
Σ%
(b)
5
pCtca
40
40
20
20
0
0
0
1
2
3
4
5
6
7
8
9
0
1
2
pCcit
3
4
pCtca
Figure 6. Sequestration diagrams of 1,2,3-tricarballylate towards
alkyltin(IV) (RSn) at t = 25 ◦ C and CRSn = 10−8 mol l−1 . = CH3 Sn3+ ;
◦ = (CH3 )2 Sn2+ ; = (CH3 )3 Sn+ . (a) pH = 5; (b) pH = 6.
(c)
100
80
Σ%
60
40
20
0
0
1
2
3
4
5
6
7
8
9
pCcit
Figure 5. Sequestration diagrams of citrate towards alkyltin(IV) (RSn) at
t = 25 ◦ C and CRSn = 10−8 mol l−1 . = CH3 Sn3+ ; ◦ = (CH3 )2 Sn2+ ;
= (CH3 )3 Sn+ . (a) pH = 5; (b) pH = 6; (c) pH = 8.
and we obtained the value of b = 0.87±0.03, for all the considered
metal ions. The equations for each metal considered are the
following:
log K(CH3 )2 Sn2+ – L
=
1.1(±0.1) + (0.87 log KM2+ – L )
for Cu2+
log K(CH3 )2 Sn2+ – L
=
2.0(±0.2) + (0.87 log KM2+ – L )
for Ni2+
=
1.9(±0.2) + (0.87 log KM2+ – L )
for Zn2+
log K(CH3 )2 Sn2+ – L
=
2.1(±0.2) + (0.87 log KM2+ – L )
for Co2+
log K(CH3 )2 Sn2+ – L
=
1.4(±0.3) + (0.87 log KM2+ – L )
for Pb2+
log K(CH3 )2 Sn2+ – L
=
3.2(±0.2) + (0.87 log KM2+ – L )
for Ca2+
These equations are useful to quantitatively predict the stability
of (CH3 )2 Sn–L complex species by using the stability data of
complexes formed by the interaction of other and, in general,
more investigated divalent metal ions with the same carboxylate
ligands. Some examples confirm the validity of these relationships:
using the log KCo(cit) = 6.47 (I = 0 mol l−1 ) we obtain the
corresponding log K(CH3 )2 Sn2+ −cit = 7.73 calculated using the
parameters of the above equation referring to the Co–L systems
(a = 2.1 ± 0.2 and b = 0.87 ± 0.03), which is very close to
the value of 7.71 experimentally determined,[11] and corrected
for I = 0 mol l−1 . Again, the log K = 3.01 for the (CH3 )2 Sn–ac
system (I = 0 mol l−1 ) is in excellent agreement with the value
log K(CH3 )2 Sn−ac = 3.02 calculated using the constant value
of the Cu–ac system and the relative empirical parameters
(a = 1.1 ± 0.1 and b = 0.87 ± 0.03) of the relationship reported
above.
c 2007 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
37
Appl. Organometal. Chem. 2008; 22: 30–38
log K(CH3 )2 Sn2+ – L
C. De Stefano et al.
Conclusions
The results reported here give a comprehensive picture of
the stability of the organotin–carboxylate systems. The large
number of systems investigated allowed us to obtain relationships
between the stability and (i) the number of both protonated
and unprotonated carboxylic groups, and (ii) other potential
O-donor ethereal or hydroxo groups present in the ligand
molecules. The obtained equations show a good correlation
between the experimental values of thermodynamic stability
parameters and the calculated ones. Predictive relationships were
also inferred to obtain stability data of diorganotin complexes
from carboxylate systems containing other most common divalent
metal ions. Finally, the sequestering capacity of carboxylate ligands
towards organotin cations was defined as function of the ligand
concentration in the pH range of interest of natural aqueous
systems. Results reported here and in the first part[12] of this short
series can be used for a better understanding of the speciation of
carboxylate systems which are widely present in most of natural
fluids.
Acknowledgements
We thank the Universities of Messina and Palermo for financial
support.
References
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Appendix
List of symbols used in this work
log β
log βn
nH
ncarbox
zcation
Y
F1 , F2 , F3
0
TS
n
%
L
pL
pL50
S
a, b
log of overall formation constant
log of overall formation constant of the
species n protonated
number of protons in the species
number of carboxylic groups of the ligand
charge of the alkyltin(IV) cation
in turn is log β or TS0
empirical functions of equation (8)
entropic factor of the species n protonated
total percentage of alkyltin(IV) cation
complexed
symbol used to indicate ligands
− log of analytical concentration of the
ligand in solution
− log of analytical concentration of the
ligand necessary to sequester 50% of the
metal ion
slope in the flex of the function ( %) vs pL
empirical parameters of equation (10)
38
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Copyright Appl. Organometal. Chem. 2008; 22: 30–38
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